1. Technical Field
The present invention relates to an electrophoretic particle, a method for producing an electrophoretic particle, an electrophoretic dispersion, an electrophoretic sheet, an electrophoretic device, and an electronic apparatus.
2. Related Art
In general, it is known that when an electric field is applied to a dispersion containing a liquid and fine particles dispersed therein, the fine particles move (electrophoretically migrate) in the liquid because of the Coulomb force. This phenomenon is referred to as electrophoresis. In recent years, electrophoretic displays, which display desired information (an image) using electrophoresis, have been attracting attention as novel displays.
The electrophoretic displays have advantages such as an image memory effect in the absence of voltage, a wide viewing angle, and the ability to display a high-contrast image at low power consumption.
The electrophoretic displays are non-emissive devices and therefore are gentler to eyes as compared with emissive display devices such as Braun tubes.
Such an electrophoretic display is known to include an electrophoretic dispersion which contains a solvent and electrophoretic particles dispersed therein and which is placed between a pair of substrates each having an electrode.
In the electrophoretic dispersion, the electrophoretic particles are those positively charged particles and those negatively charged. This allows desired information (an image) to be displayed when a voltage is applied between the substrates (electrodes) (refer to, for example, JP-A-2007-192945).
Positively charged electrophoretic particles are produced, for example, as described below.
First, particles are coated with a particulate metal oxide with a pH of more than 7, whereby base particles having a coating layer are prepared. The base particles are surface-grafted by various methods, whereby the positively charged electrophoretic particles are produced so as to have polymer molecules attached to the surfaces thereof.
Negatively charged electrophoretic particles can be produced in substantially the same manner as that used to produce the positively charged electrophoretic particles except that base particles having a coating layer are prepared using a particulate metal oxide with a pH of less than 7 instead of the particulate metal oxide with a pH of more than 7.
However, electrophoretic particles which are obtained as described above and which are positively or negatively charged tend to have a charge that depends on the density and molecular weight of the polymer molecules attached to the surfaces thereof.
That is, when the density of the attached polymer molecules is high, the charge of the electrophoretic particles tends to be low. In contrast, when the density of the attached polymer molecules is low, the charge of the electrophoretic particles tends to be high.
Therefore, in the case of increasing the density of the attached polymer molecules in consideration of the dispersibility of the electrophoretic particles in an electrophoretic dispersion, the charge of the electrophoretic particles is reduced; hence, there is a problem in that the mobility of the electrophoretic particles in the electrophoretic dispersion is low when a voltage is applied between a pair of electrodes.